DETERMINATION OF MOLECULAR WEIGHT
the thermometer inserted, and the apparatus weighed again.
The melting-point of the phenol must now he ascertained.
Warm the metal over a small flame on a sand-bath so as to
melt the phenol, leaving, however, a few crystals floating in the
liquid, and place the vessel in the cylinder, at the bottom of
which is a wire spring or pad of cotton wool. A perforated
cork at the top keeps the stem of the thermometer in position.
Let the phenol cool down well below its freezing-point, and
then shake the cylinder until solidification commences. This
will give a first approximation to the freezing-
point. The -phenol is now warmed gently as before
until only a few crystals remain unmelted. The
vessel is replaced in the cylinder and the liquid
cooled 0*5° to i° below the point previously ascer-
tained. It is now shaken until crystallisation sets
in, and then occasionally until the maximum point
is reached. The operation is repeated as often
as requisite. The substance is now introduced, a
sufficient quantity being taken to produce a depres-
sion of at least 0*5°. In order to effect this the
phenol is melted and the neck warmed with a
small flame until the thermometer is loosened and
can be withdrawn. As much phenol as possible
is allowed to drain off the neck and off the ther-
mometer, and the weighed quantity of substance
introduced. The thermometer is replaced, and any phenol which
may have run out is wiped off from the outside of the vessel, which
is then re-weighed. The freezing-point is determined as before.
The Ebullioscopic or Boiling-point Method
(Raoult).—The boiling-point of a liquid is found to be affected
by the presence of a dissolved substance in a similar manner
to the freezing-point, that is, the boiling-point of a given quantity
of a liquid is raised the same number of degrees by dissolving in
it the same number of molecules of different substances, or, in
other words, such weights of these substances as represent the
ratio of their molecular weights. These facts were first clearly
demonstrated by Raoult.
Statical Method.—The most convenient form of apparatus
for determining molecular weight by this method is Beckmann's
boiling-point apparatus shown in Fig. 32.